Send one reply to the person who is harassing you, saying in a firm tone that this reply constitutes an order to cease and desist. Tell them that you consider their attacks to be harassing, threatening and libelous and they are not to contact you again.
Print copies of the harassing emails and take them to your local police department. Explain that you feel threatened by the person and ask that your complaint be put on record.
Consider filing a restraint order. If the person harassing you lives in the same town, county or state, you might have legal options to get them to stop.